Nice job, you should be pleased!

I think you might be right, it's a combination of a lot of power and not enough nose weight, even with a 400 mine is pretty skittish and takes a lot of corrections to keep on the straight and narrow, I don't know about the extra turn fin as this will increase drag and you might find it wont turn in either direction well as it is supposed to pivot around it.

I'm curious, what's the all up weight, if you are well short of the recommended 550g, I don't see why you can't ballast it up a little in the nose or even up to say 600g all over to calm it down a little, it's not as if you lack power to move the mass!!!

If it's any help my 2/3A 9.6v cell pack weighs 175g, I expect your Lipo's are half that AND higher voltage (11.1v right) so more weight can't hurt

How high is the heat from the motor and esc after the run?